Reinforcing cap for moving coil loudspeakers



March 21, 1967 D. c. WARD 3,310,131

REINFORCING CAP FOR MOVING con LOUDSPEAKERS Filed June 11, 1964 FIG. '1. IA 2 United States Patent Office 3,310,131 Patented Mar. 21, 1967 3,310,131 REINFORCING CAP FOR MOVING COIL LOUDSPEAKERS Dennis Charles Ward, Iver, England, assignor to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Filed June 11, 1964, Ser. No. 374,447 Claims priority, application Great Britain, Jan. 30, 1964, ,925/64 6 Claims. (Cl. 181-32) The present invention relates to loud-speakers of the moving coil type, and has especial but not exclusive reference to elliptical loudspeakers.

In elliptical coned speakers, there is often a tendency 'for harmonic distortion to arise due to the cone folding across its minor axis. Similar harmonic distortion can also arise in other shapes of speaker cones especially if high power handling capabilities are required. An object of the present invention is to reduce such distortion.

In accordance with the present invention there is provided in an electro-acoustic device of the moving coil type, an elliptical conical diaphragm, an opening in the diaphragm, a moving coil affixed to the diaphragm around said opening, a cap afiixed to the diaphragm so as to close said opening, said cap comprising a layer of stifif material formed with a plurality of apertures, said layer having greater rigidity than the material of the diaphragm, and a layer of air pervious dust excluding material applied to said first layer. By reason of the fact that the cap has greater rigidity than the material of the diaphragm, it reduces the tendency for the elliptical conical diaphragm to fold across its minor axis and thereby produce harmonic distortion. Although the cap has rigidity as aforesaid, it is made to have low accous-tic impedance by reason of the apertures therein, the passage of air through said apertures not being prevented by the air pervious layer applied to the relatively rigid layer, the air pervious layer serving however to exclude the pas-sage of dust.

The present invention takes advantage of the known practice of providing a dust cover over the end of the speech coil former of a moving coil loudspeaker to protect the magnetic gap within which the speech coil moves from being fouled by dust particles and foreign bodies, especially those of a ferromagnetic nature. Hitherto, having regard to the fact that the dust cap should not exhibit a large acoustic impedance and thereby damp the movement of the cone, known covers or caps have often been constructed of a thin cotton fabric, which although it may be stiffened has not appreciably increased the rigidity of the cone.

In order that the present invention may be clearly understood and readily carried into effect, it will now be described with reference to the accompanying drawings in which:

FIGURE 1 shows a plan view of a loudspeaker fitted with a dust excluding cap,

FIGURE 2 is a cross section at A-A of FIGURE 1, and

FIGURE 3 is an exploded perspective view of the dust excluding cap.

Referring to FIGURE 3 it will be seen that the cap 1 is formed in two parts, 1a and 1b. The part 1a comprises a sheet of thin cotton fabric, and the part 1b a thin aluminium sheet of .004 inch thickness, perforated with 5 inch holes at a rate of 32 per square inch. The parts 1a and 1b are stuck together with a suitable adhesive, and formed as shown in FIGURE 2 to fit over the end of the voice coil former 6.

In FIGURE 2 the cap 1 is shown in mounted position on a conventional permanent magnet moving coil loudspeaker. The magnet system comprising part-s 11 and 12 defines a gap in which the voice coil 7 upon the former 6 operates (the size of the gap 13 has been exaggerated for the sake of clarity). The former 6 is attached to an elliptical cone diaphragm 2, and is restrained from lateral movement by a spider 8, while the periphery of the cone 2 is secured to the frame 3, being isolated by means of corrugations 14 or other means such as are well known in the art. A sealing surround 4, composed for example of cork, is applied to the mounting face of the frame, and the loudspeaker is provided with mounting holes 5. The construction of a dust excluding cap above described is such that the dust cap has greater rigidity than the cone material so that there is less tendency than would be the case in the absence of the dust cap for the cone to fold across its minor axis and thereby produce harmonic distortion. Moreover, the perforations in the aluminum sheet, being covered by an air pervious fabric provide a desirably low acoustic impedance, by allowing an air path at low frequencies.

Although the present invention has been described with reference to a particular type of loudspeaker, and although the construction of the cap has been particularised, the invention is not limited to such arrangements, as any moving coil loudspeaker may be fitted with a cap having a rigidity similar to or greater than that of the core material, providing that the cap is designed to permit the passage of air at low frequencies. Moreover a loudspeaker having a self-supporting voice coil may be utilised, the voice coil former not being an essential feature.

What I claim is:

1. In an electro-acoustic device of the moving coil type, an elliptical conical diaphragm, an opening in the diaphragm, a moving coil afiixed to the diaphragm around said opening, a cap affixed to the diaphragm so as to close said openin said cap comprising a layer of stiff material formed with a plurality of apertures, said layer having greater rigidity than the material of the diaphragm, and a layer of air pervious dust excluding material applied to said first layer.

2. An electro-accust-ic device according to claim 1 wherein said stiff material is aluminium.

3. An electro-acoustic device according to claim 2 wherein said layer of aluminium is of the order of 0.004 inch thick.

4. An electro-acoustic device according to claim 3 wherein between 30 and 34 holes per square inch are formed in said layer of aluminium and said holes are circular and of the order of inch (0.062 inch) in diameter.

5. An electro-acoustic device according to claim 1 wherein said layer of air pervious dust excluding material is a layer of cotton fabric.

6. An electro-acoustic device according to claim 1, wherein said layer of air pervious dust excluding material comprises fabric adhering to the outer surface of said layer of stiff material.

References Cited by the Examiner UNITED STATES PATENTS 2,107,757 2/1938 Kinsley 18132 X 2,641,329 6/1953 Levy et al. l81-32 2,890,760 6/1959 BObb 1813l 3,093,207 6/1963 Bozak 181-32 STEPHEN I. TOMSKY, Printary Examiner. LEO SMILOW, LOUIS I CAPOZI, Examiners. 

1. IN AN ELECTRO-ACOUSTIC DEVICE OF THE MOVING COIL TYPE, AN ELLIPTICAL CONICAL DIAPHRAGM, AN OPENING IN THE DIAPHRAGM, A MOVING COIL AFFIXED TO THE DIAPHRAGM AROUND SAID OPENING, A CAP AFFIXED TO THE DIAPHRAGM SO AS TO CLOSE SAID OPENING, SAID CAP COMPRISING A LAYER OF STIFF MATERIAL FORMED WITH A PLURALITY OF APERTURES, SAID LAYER HAV- 